Wright: Color Inheritance in Mammals 



35 



in which it differs from black of Berk- 

 shire. Let us suppose that there is a se- 

 ries between complete extension of black 

 over the coat and restriction of black 

 to the eyes, in which black splotched with 

 red, and red-spotted with black, are in- 

 termediates (factors of class 2a2) and 

 another series, inherited wholly inde- 

 pendently, of grades of dilution of red, a 

 kind of dilution (class lb) in which 

 black is not appreciably affected. On 

 this view Yorkshire white and the white 

 points of Berkshires are due to extreme 

 dilution of self red and red splotches 

 respectively and cannot be compared 

 with the white of the Hampshires. It 

 should be impossible to transfer the 

 Berkshire or Yorkshire kind of white to a 

 red breed with the production of reds 

 with Berkshire markings or whites with 

 occasional red spots. The white splotch- 

 ing of Berkshire- Yorkshire F2 hybrids 

 resemble closely in character the red or 

 sandy splotching of Tarn worth-Berk- 

 shire hybrids and, indeed, purebred 

 Berkshires and Poland-Chinas formerly 

 showed sandy splotching on their sides. 



PECULIAR RESULTS 



In attempting to apply this hypoth- 

 esis in detail certain difficulties must 

 be met. Spillman^ showed that all 

 blacks are not alike in their behavior 

 toward Tamworth or Duroc red. Hamp- 

 shire by Tamworth produces self black 

 (except for the white belt) while Berk- 

 shire or Poland China by Tamworth 

 produces a mixture of black and red like 

 tortoise-shell guinea-pigs. This, how- 

 ever, is not wholly unexpected since 

 Berkshire and Poland Chinas visibly 

 show less perfect extension of black than 

 Hampshires (again overlooking the belt). 



A more surprising difference comes out 

 between the behavior of Tamworth or 

 Duroc red and Yorkshire white in crosses 

 with the black breeds. In both of 

 these, black is either absent, or present 

 only in small spots, and one would expect 

 them to contain the same restriction 

 factor for black. Yet where Hampshire 

 by Tamworth produces full extension of 

 black, Hampshire by Yorkshire accord- 



ing to Simpson^*^ produces full restriction 

 (white). vSimilarly, Berkshire by Tam- 

 worth produces partial restriction (black- 

 red) while Berkshire by Yorkshire pro- 

 duces full restriction (white). Simpson 

 and Spillman have reported on some 

 very interesting crosses which tend to 

 show clear-cut segregation between some 

 factor in Tamworths and an allelomorph 

 in Yorkshires which produce effects in 

 the extension series. Thus Simpson^^ 

 crossed a pure Hampshire with two roan 

 sows which were ^ Tamworth, 3 4 York- 

 shire. The young were 5 black, 4 white, 

 3 red roan, and 4 red, all with more or 

 less of the Hampshire belt. These may 

 be looked on as 5 fully extended like Fi 

 Hampshire by Tamworth, and 11 fully 

 restricted (or nearly so), in this respect 

 like Fi Hampshire by Yorkshire. Spill- 

 man ^^ reports a somewhat similar cross 

 between Poland-China and Yorkshirc- 

 Tamworth hybrids. This cross pro- 

 duced 2 black-and-white (of which one 

 had as much black as a Poland-China) 3 

 black-and-red, 2 self white (or nearly so) 

 and 2 self red (or nearly so) . These may 

 be divided into 5 partly extended like Fi 

 Poland-China by Tamworth and 4 fully 

 or nearly fully restricted like Fi Poland- 

 China by Yorkshire. 



While these results would hardly be 

 expected a priori on the hypothesis of 

 independent extension and dilution 

 series, they are even more difficult to 

 explain on the other theories. If York- 

 shire white were simply a dominant 

 white in the usual sense, there is no ap- 

 parent reason why self reds should 

 appear in either of these crosses. We 

 have seen that Hampshire black is fully 

 dominant over red of Tamworth, that 

 Poland-China black is partially domi- 

 nant over the latter and finally, that 

 Yorkshire has no tendency to transmit 

 red in crosses with black breeds (Poland- 

 China and Berkshire, at least), yet in 

 the crosses above pure Hampshire or 

 Poland China sires have solid red off- 

 spring. On Spillman's hypothesis that 

 black, red and white make up a series of 

 three allelomorphs the results are equally 

 inexplicable. Both crosses must be of 



9 Spillman, W. ]. 

 '" Simpson, Q. I. 



1907. Loc.cit. 1' vSimpson, 0- I- 1914. Lor. cit. 



1911. Loc.cit. 12 Spillman, W. J. 1906. Loc.cit. 



